Hydrologic Impacts of Afforestation in Uruguay, A 20-Year Paired Watershed Study

Highlights A 20-year paired watershed study clearly showed that afforestation of a grassland watershed reduced water yield. Cumulative water yield over the 17-year tree growth period was reduced by 35%. Reductions of water yield were not evident until the fourth year after planting. Flow rates were reduced by 40% to 50% for high flows (less than 20% of the time) and by 50% to 70% for the remaining low flows. ABSTRACT. Substantial portions of grassland are being converted to managed forest in Uruguay. Long-term paired watershed studies in other continents indicate that water yields from managed forest lands are reduced compared to grasslands. Few such studies have been conducted in South America. This paired watershed study was initiated in 2000 to determine the hydrologic impacts of changing land use from grassland to pine plantation in Uruguay. Outflow rates, water table depths, rainfall, and meteorological variables were continuously measured for a 20-year period on two adjacent watersheds, LC1 (69 ha) and LC2 (108 ha), at La Corona estancia located in the Tacuarembó River basin in northern Uruguay. During the three-year pretreatment period, both watersheds remained in pasture, the traditional land use in the region. In July 2003, one watershed (LC2) was planted with loblolly pine (Pinus taeda L.), while the other (LC1) remained in pasture. Afforestation decreased cumulative water yield over the 17-year period from planting, July 2003, to June 2020 by 35%. Annual water yields from LC2 were reduced, with the magnitude of reduction dependent on weather patterns and stage of tree growth. Differences in water yields between LC1 and LC2 were significant (p < 0.01) in the last twelve years of the study compared to the first 4 years. Changes in water yield were not detected until four years after planting. Analysis of daily flow duration data indicated that flow rates after planting on LC2 were reduced between 40% and 50% for high flows occurring less than 20% of the time and reduced by 50% to 70% for the remaining low flows. Our results are consistent with other research in areas with similar rainfall. Water table and cumulative outflow data suggest that the water yield reduction is associated with deeper-rooted pine having access to more water in the soil profile for transpiration, as compared to shallow-rooted grasses. Research continues on this site to quantify the hydrologic impacts of afforestation through the production cycle for pine in Uruguay. Keywords: Cumulative outflow, ETo, Flow duration curves, Forest hydrology, Loblolly pine, Water table, Water yield.